Sri venkateswara college of engineering. Department of ECE. EC Electronic Circuits II. 2 mark questions unit wise. UNIT I Feedback Amplifiers

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1 Sri venkateswara college of engineering Department of ECE EC Electronic Circuits II 2 mark questions unit wise UNIT I Feedback Amplifiers 1. Define feedback? A portion of the output signal is taken from the output of the amplifier and is combined with the normal input signal. This is known as feedback. 2. Define positive feedback? If the feedback signal is in phase with input signal, then the net effect of the feedback will increase the input signal given to the amplifier. This type of feedback is said to be positive or regenerative feedback. 3. Define negative feedback? If the feedback signal is out of phase with the input signal then the input voltage applied to the basic amplifier is decreased and correspondingly the output is decreased. This type of feedback is known as negative or degenerative feedback. 4. Define sensitivity? Sensitivity is defined as the ratio of percentage change in voltage gain with feedback to the percentage change in voltage gain without feedback. daf/a=( da/a)(1/1+βa) 5.What are the types of feedback? i. Voltage-series feedback ii. Voltage-shunt feedback iii. Current-series feedback

2 iv. Current-shunt feedback 6. What are the basic amplifiers? The basic amplifiers are i. Voltage amplifier ii. Current amplifier iii. Transconductance amplifier iv. Transresistance amplifier. 7. What are the components of feedback amplifier? The components are i. sampling network, ii. Feedback network, and iii. mixer network. 8. What are two types of sampling? i.voltage sampling or node sampling ii.current sampling or loop sampling 9. State the two types of mixing? Series mixing Shunt mixing 10. What is transfer gain? It is the ratio of the output signal to the input signal. It is denoted by A A=Xo/Xi 11. List out the characteristics of feedback amplifier? Desensitivity Nonlinear distortion Noise distortion Frequency distortion

3 12. What is the effect of input resistance due to series mixing? The input resistance increases due to series mixing irrespective of the type of sampling. The feedback signal opposes the source signal and the input current decreases and due to this input resistance increases. Rif>Ri Where Rif = input resistance with feedback Ri = input resistance without feedback 13. What is the effect of input resistance due to shunt mixing? The input resistance decreases due to shunt mixing irrespective of the type of sampling. The feedback signal opposes the source signal and the input current decreases as a consequence Voltage reduces leading to a reduction in input resistance. Rif<Ri; Where Rif = input resistance with feedback Ri = input resistance without feedback 14. What happens to output resistance due to current sampling? The output resistance increases due to current sampling. Rof>Ro Rof = input resistance with feedback Ro= input resistance without feedback 15. What happens to output resistance due to voltage sampling? The output resistance decreases due to current sampling. Rof<Ro Rof = input resistance with feedback Ro= input resistance without feedback 16. What happens to output resistance due to current sampling? The output resistance increases due to current sampling. Rof>Ro Rof = input resistance with feedback

4 Ro= input resistance without feedback 17. Write the expression for input and output resistance of voltage series feedback amplifier. Input resistance with feedback, Rif = Ri(1+βA) Output resistance with feedback, Rof = Ro(1+βA) 18. Give the properties of negative feedback. i. Negative feedback reduces the gain ii. Distortion is very much reduced 19. Give the effect of negative feedback on amplifier characteristics. Characteristics Types of feedback Current-series Voltage-series Voltage-shunt Current-shunt Voltage gain Decreases Decreases Decreases Decreases Bandwidth Increases Increases Increases Increases Input resistance Increases Increases Decreases Decreases Output Resistance Increases Decreases Decreases Increases 20. What are the effects of negative feedback? (Or) What are the advantages of negative feedback? 1. It improves the stability of the circuit. 2. It improves the frequency response of the amplifier. 3. It improves the percentage of harmonic distortion. 4. It improves the signal to noise ratio (SNR). 5. It reduces the gain of the circuit.

5 21.Define the feedback factor β? It is the ratio between the feedback voltages to the output voltage of the amplifier. β = Vf/ Vo Where, β is a feedback factor (or) feedback ratio, Vf is the feedback voltage, Vo is the output voltage. 23). Define Desensitivity D? Desensitivity is defined as the reciprocal of sensitivity. It indicates the factor by which the voltage gain has been reduced due to feedback network. Desensitivity factor (D) = 1+A β. Where, A = Amplifier gain.. β = Feedback factor. 23) Define loop gain. (Or) What is meant by return ratio? The signal Vi is multiplied by A in passing through the amplifier is multiplier by β. In transmission through the feedback network and is multiplied by -1 in the mixer. The product of - A β is called the loop gain (or) return ratio. 24). What is the nature of input and output resistance in negative feedback? (1) Voltage series feedback: Input impedance: Zif = Zi / (1+A β) Output impedance: Zof = Zo / (1+A β) (2) Voltage shunt feedback: Input impedance: Rif = Ri * (1+A β) Output impedance: Zof = Zo * (1+ A β) (3) Current series feedback: Input impedance: Rif = Zi / (1+A β) Output impedance: Zof = Zo / (1+A β)

6 (4) Current shunt feedback: Input impedance: Rif = Ri / (1+A β) Output impedance: Rof = Ro / (1+A β) UNIT II Oscillators 1. How does an oscillator differ from an amplifier? Oscillators Amplifiers 1. They are self-generating circuits. They generate 1. They are not self-generating waveforms like sine, square and triangular circuits. They need a signal at the input and waveforms of their own. Without having input they just increase the level of the input signal. waveform. 2. It have infinite gain 2. It have finite gain 3. Oscillator uses positive feedback. 3. Amplifier uses negative feedback. 2.What is an Oscillator? An Oscillator is a Circuit, which generates an alternating voltage of any desired frequency. It can generate an a.c output signal without requiring any externally applied input signal. 3. What is Barkhausan criterion? The conditions for oscillator to produce oscillation are given by Barkhausan criterion. They are: (i). the total phase shift produced by the circuit should be 360 o or 0 o (ii).the Magnitude of loop gain must be greater than or equal to 1. 1 β. i.e. A

7 3. What are the types of sinusoidal oscillator? Mention the different types of sinusoidal oscillator? 1. RC phase shift Oscillator. 2. Wein Bridge Oscillator. 3. Hartley Oscillator 4. Colpitts Oscillator 5. Crystal Oscillator 4. Name two low frequency Oscillators. 1. RC phase shift. 2. Wein bridge Oscillator. 5. Name two high frequency Oscillators. i. Hartley Oscillator ii. Colpitts Oscillator iii.crystal Oscillator 6.What did you understand by the term stability of an Oscillator? (Or) ii). Define stability of an Oscillator. (Or) iii). Why is amplitude stabilization required? The frequency stability of an Oscillation is a measure of its ability to maintain the required frequency as precisely constant as possible over a long period of time interval. 7. What are the essential parts of an Oscillator? i. Tank circuit (or) Oscillatory circuit. ii.. Amplifier (Transistor amplifier) iii.. Feedback Circuit. 8. What are the advantages of Rc phase shift Oscillator? i. It is best suited for generating fixed frequency signals in the audio frequency range. ii.simple Circuit. Iii Pure sine wave output is possible. 9. What is piezo electric effect? The piezo electric Crystals exhibit a property that if a mechanical stress is applied across one face the electric potential is developed across opposite face. The inverse is also live. This phenomenon is called piezo electric effect. 10. Draw the equivalent circuit of a Crystal. Where R = internal frictional losses.

8 L = Mass of the crystal. Cs = Stiffness of the crystal. Cp = Shunt capacitance. 11. List the disadvantages of crystal Oscillator. 1. It is suitable for only low power circuits 2. Large amplitude of vibrations may crack the crystal. 3. It large in frequency is only possible replacing the crystal with another one by different frequency. 12. What is meant by resonant Circuit Oscillators? LC Oscillators are known as resonant circuit oscillator because the frequency of operation of LC Oscillator is nothing but a resonant frequency of tank circuit or LC tank circuit produces sustained Oscillation at the resonant circuit oscillator. 13 Classify the different types of Oscillators. i. According to waveform generation a. Sinusoidal Oscillator. b. Relaxation Oscillator. ii. According to the fundamental mechanism involved a. Negative resistance Oscillator. b. Feedback Oscillator. iii. According to frequency generated a. Audio frequency Oscillator (Up to 20KHZ) b. Radio frequency Oscillator (20KHZ) c. Very high frequency oscillator (30 MHZ to 300 MHZ). d. Ultra high frequency Oscillator (300 MHX to 3 GHZ). e. Microwave frequency Oscillator (> 3 GHZ). iv. According to type coupling.

9 a. LC Oscillator. b. RC Oscillator. 14. Why RC phase shift is needed in a RC phase shift Oscillator? The amplifier used causes a phase shift of 180 o than the feedback network should create phase shift of 180 o, to satisfy the Barkhausen Criterion. Hence in a phase shift oscillators, three sections of RC circuit are connected in cascade, each introducing a shift of 60 o, thus introducing a total phase shift of 180 o, due to feedback network. 15 What are the advantages of crystal Oscillators over other Oscillator? To maintain the output frequency of an oscillator at a constant value, a crystal may be used to control the frequency of oscillation. 16. What is negative resistance? Name a few devices which offer this resistance. It is defined as the resistance of a device which offers when operated in resistance region. the negative Ex: - Tunnel diodes, UJT... Etc., 17 Define gain and phase Margin. Gain Margin: It is defined as the value of /Aβ/ in decibels at the frequency at which the phase angle of Aβ is 180 o, negative gain margin signifies decibel rise in open loop gain a theoretical possibility without oscillation. A positive gain margin signifies that amplifier is potentially unstable. Phase Margin: It is defined as 180 o minus the Magnitude of angle of Aβ at the frequency at which /Aβ/ is unity. 18. What is a beat frequency oscillator? Beat frequency Oscillator (BFO) is an Oscillator in which a deserved signals frequency such as the beat frequency produced by combining the different signal frequencies such as on different radio frequencies.

10 1. What is meant by tuned amplifiers? UNIT III Tuned amplifiers Tuned amplifiers are amplifiers that are designed to reject a certain range of frequencies below a lower cut off frequency ωl and above a upper cut off frequency ωh and allows only a narrow band of frequencies. 2. Classify tuned amplifiers. 1. Single tuned amplifier. 2. Double tuned amplifier. 3. Synchronously tuned amplifier. 4. Stagger tuned amplifier. 3. What are the advantages of double tuned amplifier? In double tuned amplifiers, the tuning is done both at the primary and secondary. The double tuned amplifiers provide a wider bandwidth, flatter pass band and a greater selectivity. 4. Define resonance. The reactance of the capacitor equals that of the inductor reactance. i.e ωc. = 1 / ωl. 5. What is Quality factor? The ratio of inductive reactance of the coil at resonance to its resistance is known as quality factor. Q = XL / R 6. Define gain bandwidth product of a tuned amplifier. The gain bandwidth (GBW) product is a figure of merit defined in terms of mid band gain and upper 3-db frequency fh as GBW = Aim fh = gm / 2πc 7. What is the other name for tuned amplifier? Tuned amplifiers used for amplifying narrow band of frequencies hence it is also known as narrow band amplifier or Band pass amplifier.

11 8. What is a synchronously tuned amplifier? When tuned amplifiers are cascaded if all the amplifier stages are identical and tuned to same frequency fo then it is called as synchronously tuned amplifier. This results in a increased in gain and reduction in bandwidth. 9. What is meant by neutralization? It is the process by which feedback can be cancelled by introducing a current that is equal in magnitude but 180 o out of phase with the feedback signal at the input of the active device. The two signals will cancel and the effect of feedback will be eliminated. This technique is termed as neutralization. 10. What is unilateralisation? It is the phenomenon by which a signal can be transmitted from the input to the output alone and not vice versa. In a unilateralised amplifier both resistive and reactive effects are cancelled. 11. What is stagger tuned amplifier? In this configuration one or more tuned amplifiers are cascaded each amplifier stage is tuned to different frequencies. This results in decreased gain and increased bandwidth. 12. What is the effect of Q on stability? Higher the value of Q provides better selectivity, but smaller bandwidth and larger gain. Hence it provides less stability. 13. What is the application of tuned amplifiers? The application of tuned amplifiers to obtain a desired frequency and rejecting all other frequency in (i). Radio and T.V broadcasting as tuning circuit. (ii). Wireless communication system. 14. What is meant by unloaded and loaded Q of tank circuit? [APR 2003] Unloaded Q is the ratio of stored energy to dissipated energy in a reactor or resonator. The loaded Q (or) QL of a resonator is determined by how tightly the resonator is coupled to its terminations. 15. Mention the applications of class c tuned amplifier.[apr 2003]

12 One of the most common applications for mixer is in radio receivers. The mixer is used to convert incoming signal to a lower frequency where it is easier to obtain the high gain and selectivity required. Mixer circuits are used to translate signal frequency to some lower frequency or to some higher frequency. When it is used to translate signal to lower frequency it is called down converter. When it is used to translate signal to higher frequency, it is called up converter. 16. Mention the need for stagger-tuned amplifier. The double tuned amplifier gives greater 3 db bandwidth having steeper sides and flat top. But alignment of double tuned amplifier is difficult. To overcome this problem two single tuned amplifiers are cascaded. 17. What are the advantages of tuned circuit? High selectivity Smaller collector supply voltage Small power gain. 18. Mention the bandwidth of a double tuned amplifier. Bandwidth (ω2 ω1) = ωo / Q (b 2 1) + 2b Where, ωo is the resonance frequency in cycle per sec. Q is the Quality factor of the coil alone. B is a constant. 19. What is principle of Hazel tine neutralization? Hazel tine introduced a circuit in which the troublesome effect of the collector to base capacitance of the transistor was neutralized by introducing a signal which cancels the signal coupled through the collector to base capacitance. 20. List the performance measure of a tuned amplifier. Selection of a desired radio frequency signal. Effective quality factor. Gain Bandwidth. 21. What are the characteristics of an ideal tuned amplifier? Selects a single radio frequency and amplifiers the same by rejecting all other frequencies.

13 Bandwidth is zero. Harmonic distortion is zero. 22. Write down the relationship between bandwidth and effective Q of a tuned amplifier? Bandwidth = ωo / Q effective. 23. What are the different methods of coupling? (or) Point out different methods of coupling the load to a tuned amplifier. The different methods of coupling the load to a tuned amplifier are: Capacitive coupling, Inductive coupling. 24. Why tuned amplifier cannot be used at low frequency? For low frequencies the size L and C are large. So the circuit will be bulky and expensive, hence the tuned amplifiers cannot be used at low frequency. 25. What are band pass amplifiers? Band pass amplifiers are amplifiers circuits which allow a certain range of frequencies in between two cut off frequencies (f1, f2) and attenuates all the other frequencies or rejects all other frequencies. 26. What are the drawbacks of a single tuned amplifier? Narrow bandwidth on smaller pass band, which will result in poor production of the audio signal. The sides (and the top) of a gain versus frequency curve are not steeper. 27. The band width of sing tuned amplifier is 10 KHz. If four such stages are connected in series, What is its effective bandwidth. The bandwidth of n number of tuned amplifiers connected in series is, BWT = BW1 Where, BWT = Total (effective) Bandwidth. Bw1 = Single tuned amplifier bandwidth. n = number of stages. BWT = 10*10 3

14 BWT = 43.5 KHz 28. The bandwidth of a bouble-tuned amplifier is 10 KHz. Calculate the number of such stages to be connected to obtain the bandwidth of KHz. BWT = BW1 (2 1/n - 1) 1/4 2 1/n = Taking log on both sides, 1/n log (2) = log (1.0676) n = Calculate the resonant frequency of a class c tuned amplifier whose capacitor c=10pf and inductor L=1mH. The resonant frequency of class-c tuned amplifier is fr = 1 / 2 π = 1/ 2*3.14 fr = 1.59 MHz UNIT IV Wave shaping and Multivibrator circuits 1. What is a linear waveform-shaping circuit? The process by which the shape of a nonsinusoidal signal is changed by passing the signal through the network consisting of linear elements is called Linear Wave Shaping. 2. Define integrator. Integrator is a circuit that passes low frequencies of the input and attenuates high frequencies. Integrator implies that the output voltage is an integral of the input voltage. 3. Define differentiator.

15 Differentiator is a circuit that passes high frequencies of the input and attenuates low frequencies. It implies that the output voltage is the differential of the input. 4. What is meant by clippers? The circuit with which the waveform is shaped by removing a portion of the input signal without distorting the removing part of the alternating waveform is called a clipper. 5. What is meant by clampers? Clamping network shifts (clamp) a signal to a different d.c level, i.e., it introduces a d.c level to an a.c signal.hence,the clamping network is known as d.c restorer. 6. What is meant by multivibrator? Multivibrators are two stage switching circuits in which the output of the first stage is fed to the input of the second state and vice-versa. The outputs of two stages are complementary. 7. List the types of multivibrator. 1. Astable multivibrator 2. Bistable multivibrator. 3. Monostable multivibrator. 8. Define Astable multivibrator. Astable multivibrator is a multivibrator in which neither state is stable. There are two temporary states. The circuit changes state continuously from one quasi stables state to another at regular intervals without any triggering. This generates continuous square waveform without any external signal. 9. Define monostable multivibrator. When a trigger pulse is applied to the input circuit, the circuit state is changed abruptly to unstable state for a predetermined time after which the circuit returned to its original stable state automatically. 10. Define the Bistable multivibrator. Bistable multivibrator signifies a circuit which can exist indefinitely in either of two stable states and which can be induced to make an abrupt transition from one state to other by applying an external triggering signal.

16 11.What is the use of commutating capacitors? The Commutating capacitors can be used to reduce the transition time in a low to high level and vice versa. 12. What are the features of a collector coupled Astable multivibrator? In the circuit there exists a capacitive coupling between the stages. So both the transistors cannot remain at cut off. The circuit has two quasi-stable states, between which it can make transitions. 13. What is delay time? The time required for the current to rise to 10% of its maximum (saturation) value Ics is called the delay time td. 14. What is the total turn on time? The total turn on time is ton is the sum of the delay time and rise time, ton = td + tr Where, td = Delay time. tr = Rise time. 15. What is storage time? The interval that elapses between the transition of the input waveform and the time when the collector current has dropped to 90 % of total output is called the storage time ts. 16. Define transition time. The time interval during which the conduction transfer from one transistor to another transistor is defined as transition time Define resolving time. It is the minimum time interval between two consecutive trigger pulses and equals to transition time plus the settling time. 18. What is meant by linear wave shaping circuit? The action of a linear network in producing a waveform at its outputs different from its output is known as linear wave shaping circuit. 19. What is meant by Schmitt trigger?

17 In a circuit which converts sine wave into a square wave. It also has two opposite operating states as in all multivibrator. In this case the triggering signal is a slowly varying a.c voltage. 20. Define UTP and LTP. UTP [Upper trigger point] is the point at which the transistor enters into conduction.i.e. OFF to ON state. LTP [Lower trigger point] is the point at which the transistor enters from ON to OFF state. 21. What are different types of triggering of bistable multivibrator? 1. Asymmetrical triggering. 2. Symmetrical triggering. 22. Define Symmetrical triggering. It is the method of triggering, by which pulses are applied at only one input and these are steered or directed to appropriate transistors sequentially. 23. What is meant by unsymmetrical triggering? If two signals from two separate trigger source are used, one signal to cause the change in one direction.i.e. from ON to OFF and the other signal cause change from OFF to On. It is used in logic circuit. 24. What is meant by hysteresis? The input voltage difference between UTP and LTP is known as hysteresis. 25. Define Settling time. It is defined as the time required for recharging of commutating capacitors after transfer of conduction Distinguish oscillator and multivibrator. Multivibrator operates in non-linear region of transfer characteristics and oscillator operators linear or active region of its transfer characteristics. 1. What is blocking oscillator? UNIT V Blocking Oscillators and Time base Generators The circuit which uses a regenerative feedback, producing a single pulse or pulse train is called a blocking oscillator.

18 2. Which are the two important elements of a blocking oscillator? 1. Active element like transistor. 2. A pulse transformer. 3. What is the function of pulse transformer in blocking oscillator? A pulse transformer is used to couple output of the transistor back to the input. The nature of such feedback through pulse transformer is controlled by relative winding polarities of a pulse transformer. 4.What is pulse transformer? A pulse transformer is basically a transformer which couples a source of pulses of electrical energy to the load, keeping the shape and other properties of pulses unchanged. The voltage level of the pulse can be raised or lowered by designing the proper turn s ratio for the pulse transformer. 5. State the features of pulse transformer. 1. Generally iron cored and small in size. 2. The leakage inductance is minimum. 3. The interwinding capacitance is low. 4. The cores have high permeability. 5. They have high magnetizing inductance. 6. Draw the equivalent circuit of pulse transformer. Explain the various elements in it.? In this circuit, 7. What is Leading edge response? R1 = Resistance of primary winding. R2 = Resistance referred to primary. σ = Series inductance which is leakage inductance. L = Shunt inductance which is magnetizing inductance. C = Transformer and external loading capacitive effect.

19 At start there is an overshoot and then the pulse settles down. The response till it settles down after the overshoot is called leading edge response. 8. What is trailing edge response? The response generally extends below the zero amplitude after the end of pulse width is called back swing. The portion of response from backswing till it settles down is trailing edge response. 9. What is flat top response? The portion of the response between the trailing edge and the leading edge is called flat top response. 10. Define rise time of a pulse. The rise time is an important parameter related to this part of the response. It is defined by the time required by the pulse to rise from 10 % of its amplitude to 90 % of its amplitude. 11. Define the displacement error( ed ) of a sweep voltage. It is defined as the maximum difference between the actual sweep voltage and linear sweep voltage which passes through the beginning and end points of the actual sweep. It is another way of specifying the linearity of a sweep waveform. 12. Mention the application of the pulse transformer. 1. To change the amplitude and impedance level of a pulse. 2. To invert the polarity of the pulse. 3. To provide dc isolation between source and a load. 4. To differentiate a pulse. 5. for coupling the stages of a pulse amplifier 6. Also used in digital signal transmission. 13. What is current time base generator? The circuit which produces current which linearly increases with time is called current time base generator. 14. What are the applications of the blocking oscillator? 1. The blocking oscillator can be used as low impedance switch used to discharge a capacitor very quickly. 2. To produce large peak power pulses, both the types of oscillators cab be used.

20 3. The output of the blocking oscillator can be used to produce gating waveform with very low mark space ratio. 4. It may be used as frequency divider or counter in digital circuits. 15. List various sweep circuits Exponential charging circuit Constant-current charging circuit. Miller circuit boot strap circuit Inductor circuit. 16. What do you mean by voltage time base generators? Circuits used to generate a linear variation of voltage with time are called voltage timebase generators. 17. What do you mean by linear time base generator? Circuits provide an output waveform which exhibits a linear variation of voltage with time is called linear time base generators.